1. Field of the Invention
The present invention relates to a stereo microphone unit and a stereo microphone, and more particularly, the invention relates to a stereo microphone unit that can have a smaller size compared with conventional counterparts and a stereo microphone including the same.
2. Description of the Related Art
An XY stereo system and an MS stereo system are known as sound pickup systems of a stereo microphone. In the XY stereo system, two unidirectional microphone units are fixed to form an appropriate angle. The microphone unit directed to the left outputs L channel signal and microphone unit directed to the right outputs R channel signal. The appropriate angle is, for example, 120 degrees (see, for example Japanese Utility Model Application Publication H6-35597).
In the MS stereo system, a unidirectional microphone unit and a bidirectional microphone unit are used. A main signal M obtained from the unidirectional microphone unit and a directional signal S obtained from the bidirectional microphone unit directed to a direction orthogonal to that of the unidirectional microphone unit are fed to a matrix circuit to generate (M+S) and (M−S) signals. For example, the (M+S) signal is an L channel signal and the (M−S) signal is an R channel signal (see for example, Japanese Patent Application Publication 2002-374592).
FIG. 5 illustrates an example of a stereo microphone employing the XY stereo system. In FIG. 5, a stereo capacitor microphone unit 100 includes a pair of left (L) channel side unidirectional microphone unit 100L and right (R) channel side unidirectional microphone unit 100R. In FIG. 5, components other than the pair of unidirectional microphone units 100L and 100R on the left and the right side, respectively, are omitted.
As illustrated in FIG. 5, the XY stereo system has a more simple circuit configuration compared with that for the MS stereo system and thus is mainly employed in a low-cost stereo microphone
In the XY stereo system, the stereo capacitor microphone units 100L and 100R need to be fixed with their respective directional axes DL and DR forming an appropriate angle. Therefore, a holder that holds the microphone units in an appropriate angular relationship is required. In addition, to make the range of the stereo sound variable, a mechanism is required with which the angle between the directional axes DL and DR can be changed.
Generally, two microphone units in the XY stereo system are incorporated in a single head case (windshield). Such a head case is required to have a large size and a special shape to fix the two microphone units in an appropriate angular relationship.
A stereo microphone unit is known that can solve the above problems and allows an XY stereo microphone to be formed with small number of components and small size (see, for example Japanese Patent Application Publication 2008-227779). Here, two general-purpose unidirectional microphone units are fixed with their main axes forming 180 degrees. A sound insulating cover is provided over a space serving as a rear acoustic terminal between respective fixed electrodes of the left and right microphone units. Directional axes for sound pickup can be adjusted by shifting the position of the rear acoustic terminal.
An example of a stereo microphone unit disclosed in Japanese Patent Application Publication 2008-227779 is illustrated in FIG. 6. As illustrated in FIG. 6, this stereo microphone unit 200 includes: general-purpose unidirectional capacitor microphone units 200L, and 200R being fixed with their respective main axes forming 180 degrees; and a sound insulating cover 201 provided over a space formed between respective fixed electrodes of units 200L and 200R. A position offset from the main axes by the sound insulating cover 201 is a rear acoustic terminal RT. Accordingly, the stereo microphone unit has directionality capable of performing stereo sound pickup with the directional axes DL and DR forming a certain angle as illustrated in FIG. 6.
In such a stereo microphone unit disclosed in Japanese Patent Application Publication 2008-227779, an air chamber is inevitably formed between the two units (left and right) and the insulating cover. The air chamber serves not only as a rear acoustic terminal but also as a common resonator for respective rear acoustic terminals of left and right units. Therefore directional collapse occurs due to deterioration of acoustic characteristics in high frequency range and deterioration of S/N ratio.
Exemplary frequency characteristics of the stereo microphone unit 200 illustrated in FIG. 6 are depicted in FIG. 7. In FIG. 7, the horizontal axis represents frequency of a signal emitted from a sound source, and the vertical axis represents a gain in the stereo microphone unit 200. FIG. 7 depicts the L channel signal of the stereo microphone unit 200. A graph a in FIG. 7 represents a case where the sound source is at the front side in the main axis of the stereo capacitor microphone unit 200L, that is, at the diaphragm side on the main axis of the stereo capacitor microphone unit 200L (see FIG. 6). A graph b in FIG. 7 represents a case where the sound source is at the position offset by 90 degrees from the main axis of the stereo microphone unit 200, that is, at the sound insulating cover 201 side. A graph c in FIG. 7 represents a case where the sound source is at the rear side in the main axis of the stereo capacitor microphone unit 200L, that is, at the stereo capacitor microphone unit 200R side. A graph d represents a case where the sound source is at the rear acoustic terminal RT.
As depicted in FIG. 7, the stereo sound pickup is possible for a signal with frequency lower than 5 kHz because the gain in the front side in the main axis (graph a) and other gains with different sound source directions is different and sounds from left and right can be distinguished and picked up.
On the other hand, with a signal with frequency not lower than 5 kHz, resonance due to the air chamber occurs to provide a substantially omnidirectional state. When this happens, sounds from left and right cannot be distinguished, and thus, stereo sound pickup is impossible.
Such resonation can be prevented by providing an acoustic resistor in the air chamber. Unfortunately, provision of such an acoustic resistor, which has a certain amount of thickness, limits the downsizing of the stereo capacitor microphone unit as a whole.
In addition, the stereo capacitor microphone unit is likely to be affected by wind noise because each of the left and the right units has acoustic terminals respectively at the front and the back thereof. Accordingly, the stereo capacitor microphone unit needs to be improved in this point as well.